1. Field of the Invention
The present invention relates generally to the detection and kits for the detection of target molecules and, more particularly, to nucleic acid-based detection assays that produce multiple signals from a target molecule by generating multiple copies of detectable oligonucleotides through reiterative synthesis events on a defined nucleic acid template, particularly via abortive transcription initiation. The method and kits of the invention may be used to detect mutations, RNA molecules, pathogens, proteins, or pre-cancerous conditions.
2. Related Art
The development of various methods for nucleic acid detection and the detection of nucleic acid amplification products has led to advances in the detection, identification, and quantification of nucleic acid sequences in recent years. Nucleic acid detection is potentially useful for both qualitative analyses, such as the detection of the presence of defined nucleic acid sequences, and quantitative analyses, such as the quantification of defined nucleic acid sequences. For example, nucleic acid detection may be used to detect and identify pathogens; detect genetic and epigenetic alterations that are linked to defined phenotypes; diagnose genetic diseases or the genetic susceptibility to a particular disease; assess gene expression during development, disease, and/or in response to defined stimuli, including drugs; as well as generally foster advancements in the art by providing research scientists with additional means to study the molecular and biochemical mechanisms that underpin cellular activity.
Nucleic acid detection technology generally permits the detection of defined nucleic acid sequences through probe hybridization, that is, the base-pairing of one nucleic acid strand with a second strand of a complementary, or nearly complementary, nucleic acid sequence to form a stable, double-stranded hybrid. Such hybrids may be formed of a ribonucleic acid (RNA) segment and a deoxyribonucleic acid (DNA) segment, two RNA segments, or two DNA segments, provided that the two segments have complementary or nearly complementary nucleotide sequences. Under sufficiently stringent conditions, nucleic acid hybridization may be highly specific, requiring exact complementarity between the hybridized strands. Typically, nucleic acid hybrids comprise a hybridized region of about eight or more base pairs to ensure the binding stability of the base-paired nucleic acid strands. Hybridization technology permits the use of one nucleic acid segment, which is appropriately modified to enable detection, to “probe” for and detect a second, complementary nucleic acid segment with both sensitivity and specificity. In the basic nucleic acid hybridization assay, a single-stranded target nucleic acid (either DNA or RNA) is hybridized, directly or indirectly, to a labeled nucleic acid probe, and the duplexes containing the label are quantified. Both radioactive and non-radioactive labels have been used.
However, a recognized disadvantage associated with nucleic acid probe technology is the lack of sensitivity of such assays when the target sequence is present in low copy number or dilute concentration in a test sample. In many cases, the presence of only a minute quantity of a target nucleic acid must be accurately detected from among myriad other nucleic acids that may be present in the sample. The sensitivity of a detection assay depends upon several factors: the ability of a probe to bind to a target molecule; the magnitude of the signal that is generated by each hybridized probe; and the time period available for detection.
Several methods have been advanced as suitable means for detecting the presence of low levels of a target nucleic acid in a test sample. One category of such methods is generally referred to as target amplification, which generates multiple copies of the target sequence, and these copies are then subject to further analysis, such as by gel electrophoresis, for example. Other methods generate multiple products from a hybridized probe, or probes, by, for example, cleaving the hybridized probe to form multiple products or ligating adjacent probes to form a unique, hybridization-dependent product. Still other methods amplify signals generated by the hybridization event, such as a method based upon the hybridization of branched DNA probes that have a target sequence binding domain and a labeled reporting sequence binding domain.
There are many variations of target nucleic acid amplification, including, for example, exponential amplification, ligation-based amplification, and transcription-based amplification. An example of an exponential nucleic acid amplification method is the polymerase chain reaction (PCR), which has been disclosed in numerous publications. See, for example, Mullis et al., Cold Spring Harbor Symp. Quant. Biol. 51:263–273 (1986); Mullis et al. U.S. Pat. No. 4,582,788; and Saiki R. et al. U.S. Pat. No. 4,683,194. An example of ligation-based amplification is the ligation amplification reaction (LAR) which is disclosed by Wu et al. in Genomics 4:560 (1989). Various methods for transcription-based amplification are disclosed in U.S. Pat. Nos. 5,766,849 and 5,654,142; and also in Kwoh et al., Proc. Natl. Acad. Sci. U.S.A. 86:1173 (1989).
The most commonly used target amplification method is the polymerase chain reaction (PCR), which consists of repeated cycles of DNA polymerase-generated primer extension reactions. Each reaction cycle includes heat denaturation of the target nucleic acid; hybridization to the target nucleic acid of two oligonucleotide primers, which bracket the target sequence on opposite strands of the target that is to be amplified; and extension of the oligonucleotide primers by a nucleotide polymerase to produce multiple, double-stranded copies of the target sequence. Many variations of PCR have been described, and the method is being used for the amplification of DNA or RNA sequences, sequencing, mutation analysis, and others. Thermocycling-based methods that employ a single primer have also been described. See, for example, U.S. Pat. Nos. 5,508,178; 5,595,891; 5,683,879; 5,130,238; and 5,679,512. The primer can be a DNA/RNA chimeric primer, as disclosed in U.S. Pat. No. 5,744,308. Other methods that are dependent on thermal cycling are the ligase chain reaction (LCR) and the related repair chain reaction (RCR).
Target nucleic acid amplification may be carried out through multiple cycles of incubation at various temperatures (i.e., thermal cycling) or at a constant temperature (i.e., an isothermal process). The discovery of thermostable nucleic acid modifying enzymes has contributed to rapid advances in nucleic acid amplification technology. See, Saiki et al., Science 239:487 (1988). Thermostable nucleic acid modifying enzymes, such as DNA and RNA polymerases, ligases, nucleases, and the like, are used in methods that are dependent on thermal cycling as well as in isothermal amplification methods.
Isothermal methods, such as strand displacement amplification (SDA) for example, are disclosed by Fraiser et al. in U.S. Pat. No. 5,648,211; Cleuziat et al. in U.S. Pat. No. 5,824,517; and Walker et al., Proc. Natl. Acad. Sci. U.S.A. 89:392–396 (1992). Other isothermal target amplification methods include transcription-based amplification methods in which an RNA polymerase promoter sequence is incorporated into primer extension products at an early stage of the amplification (WO 89/01050), and a further, complementary, target sequence is amplified through reverse transcription followed by physical separation or digestion of an RNA strand in a DNA/RNA hybrid intermediate product. See, for example, U.S. Pat. Nos. 5,169,766 and 4,786,600. Further examples of transcription-based amplification methods include Transcription Mediated Amplification (TMA), Self-Sustained Sequence Replication (3SR), Nucleic Acid Sequence Based Amplification (NASBA), and variations there of. See, for example, Guatelli et al. Proc. Natl. Acad. Sci. U.S.A. 87:1874–1878 (1990) (3SR); U.S. Pat. No. 5,766,849 (TMA); and U.S. Pat. No. 5,654,142 (NASBA).
These and other techniques have been developed recently to meet the demands for rapid and accurate detection of pathogens, such as bacteria, viruses, and fungi, for example, as well as the detection of normal and abnormal genes. While all of these techniques offer powerful tools for the detection and identification of minute amounts of a target nucleic acid in a sample, they all suffer from various problems.
One problem, especially for PCR, is that conditions for amplifying the target nucleic acid for subsequent detection and optional quantitation vary with each test, that is, there are no constant conditions favoring test standardization. Further, amplification methods that use a thermocycling process have the added disadvantage of extended lag times which are required for the thermocycling block to reach the “target” temperature for each cycle. Consequently, amplification reactions performed using thermocycling processes require a significant amount of time to reach completion. The various isothermal target amplification methods do not require a thermocycler and are therefore easier to adapt to common instrumentation platforms. However, the previously described isothermal target amplification methods also have several drawbacks. Amplification according to the SDA methods requires the presence of defined sites for restriction enzymes, which limits its applicability. The transcription-based amplification methods, such as the NASBA and TMA methods, are limited by the need to incorporate a polymerase promoter sequence into the amplification product by a primer.
Accordingly, there is a need for rapid, sensitive, and standardized nucleic acid signal detection methods that can detect low levels of a target nucleic acid in a test sample. These needs, as well as others, are met by the inventions of this application.
All patents, patent publications, and scientific articles cited or identified in this application are hereby incorporated by reference in their entirety to the same extent as if each individual document was specifically and individually indicated to be incorporated by reference in its entirety.